Predictions of temperature rise as a consequence of global warming vary
greatly, from 0.5° C to 2.5° C by
2030 (Schneider & Rosenberg, 1989). With such uncertainty there is limited benefit in
trying to predict detailed outcomes, but two possible effects may be briefly considered.
Firstly, the direct effect of a rise in sea temperature may change the geographic
distribution of species. Secondly, any changes in sea level will affect the environment of
CFT communities. In preceding sections the influence of environmental factors, and the
regional nature of many species and biotope distributions, have both been emphasised.

Global warming can be expected to affect the geographical range of
species reaching their northern or southern limits within the British Isles. Southern
species with limited ranges in the south west such as Eunicella verrusosa (App. 4,
Fig 14a)and Balanophyllia regia (App. 4, Fig 15b)may be expected to
extend their ranges. Conversely northern species like Swiftia pallida (App. 4, Fig
14c)and Strongylocentrotus droebachiensis (App. 4, Fig 16d)may
retreat. Examples of such temperature-mediated distribution changes have been described
for intertidal species following long-term intensive monitoring - the studies on the
barnacles Chthamalus and Semibalanus in the south-west of Britain provide
the clearest example (Southward, 1967, 1991). It will require similarly detailed
monitoring to reveal such changes subtidally, but it is necessary to recognise such
naturally mediated changes as being distinct from anthropogenic impacts.

Estimates of sea level changes, as a result of global warming, are
equally highly variable. They range from 0.5m to 3.5m over the next hundred years (Boorman
et al., 1989), though most estimates are near the low end of this range (Houghton et al.,
1990). Such changes might have a marginal effect on the position of the
infralittoral/circalittoral boundary, but major direct effects on the CFT biotopes, which
cover a substantial depth range, would not be expected even from the higher estimates.
Indirect effects are more likely, where changes to the coastal environment may promote
rapid erosion with increased levels of turbidity and siltation.